Oil and gas providers are developing carbonate reservoirs of ever increasing depths to meet the ever increasing, worldwide demand for energy. Enhancing productivity from deep carbonate reservoirs presents a new challenge in the field of stimulation fluids due to the increased temperatures encountered at those deeper depths.
Acidizing or acidization is used extensively in well stimulation operations to increase the permeability of certain low permeability rocks such as carbonates, thereby facilitating the flow of oil to the wellbore. As acid is injected into the porous medium (carbonate rock), highly-permeable channels or “wormholes” are formed by the dissolution of the carbonate material. A successful matrix treatment produces thin, but deep, wormholes with a minimal amount of injected acid.
The formation of wormholes is a dynamic process that involves a balance between the acid injection rate and the reaction rate between the acid and formation rock. The reaction kinetics of a strong acid reacting with a carbonate involves three steps: (1) the transport of H+ ions from the bulk solution to the surface of the carbonate; (2) the reaction of H+ and carbonate taking place on the carbonate surface; and (3) the transport of the reaction products from the carbonate surface to the bulk solution. The slowest step controls the global reaction.
Hydrochloric acid (HCl) is commonly used for carbonate matrix acidization operations due at least in part to its relatively low cost and high dissolution capability for carbonates. However, the rapid reaction rate between the HCl and the carbonate limits the penetration into the formation.